As an example taken at random from my undergrad days, see

https://jasomill.at/metric.pdf

produced with

    \begin{align*}
      f^*(dx& \otimes dx + dy \otimes dy + dz \otimes dz)\\
      =&\ f^*(dx \otimes dx) + f^*(dy \otimes dy) + f^*(dz \otimes dz)\\
      =&\ d(f^*x) \otimes d(f^*x) + d(f^*y) \otimes d(f^*y) + d(f^*z) \otimes d(f^*z)\\
      =&\ d(x \circ f) \otimes d(x \circ f) + d(y \circ f) \otimes d(y \circ f) + d(z \circ f) \otimes d(z \circ f)\\
      =&\ df^1 \otimes df^1 + df^2 \otimes df^2 + df^3 \otimes df^3\\
      =&\ \left(\pdFOneU du + \pdFOneV dv\right) \otimes \left(\pdFOneU du + \pdFOneV dv\right)\\
      +&\ \left(\pdFTwoU du + \pdFTwoV dv\right) \otimes \left(\pdFTwoU du + \pdFTwoV dv\right)\\
      +&\ \left(\pdFThreeU du + \pdFThreeV dv\right) \otimes \left(\pdFThreeU du + \pdFThreeV dv\right)\\
      =&\ \left(\gOneOne\right) du \otimes du\\
      +&\ \left(\gOneTwo\right) \left(du \otimes dv + dv \otimes du\right)\\
      +&\ \left(\gTwoTwo\right) dv \otimes dv.
    \end{align*}

(note that, using my editor defaults, none of this equation's lines are truncated or wrapped)

The \g... and \pdF... are trivial ad hoc macros defined in the document. Producing the same document by repeatedly copy/pasting the tensor components and partial derivatives would have been considerably more time-consuming and error-prone.

Also notable is the align environment: type \\ for a manual line break and & at each point that is to be aligned.

I just tried to reproduce this with the "WYSIWYG" equation editor in Word, and I can't figure out how to do it — right-clicking on plus and equal signs gives an "Align at this Character" option, but this appears to be a special case, as the option doesn't appear when clicking on anything else in the example.

In particular, had the first line involved (implied) multiplication instead of addition, there apparently wouldn't be any acceptable point to declare alignment!?!

I'm also not sure what it means to align "at" a character, as characters have width, and characters to be aligned aren't necessarily the same width. Does it align the left sides? The right sides? The center?

Moreover, the default key bindings in the Word equation editor are counterintuitive: the usual binding for "manual line break" instead splits the equation in two and there doesn't appear to be any default binding for the "Insert Manual Break" equation editor command at all.

Sure, LaTeX has a learning curve, but it's not at all obvious to me that Word is any better in this respect.